This study will investigate whether black and white patients with hormone receptor-positive (HR+) breast cancer report adverse symptoms differently, contributing to diverse treatment decisions and ultimately disparities in health outcomes. Despite significant reductions in breast cancer mortality over the past two decades, black-white (B-W) mortality disparities continue to widen. Among the 70% of breast cancer patients that have HR+ tumors, black women have double the death rate of white women with the same diagnosis. Adjuvant endocrine therapy (AET), typically using the selective estrogen receptor antagonist tamoxifen and/or an aromatase inhibitor given orally for at least five years, significantly and substantially improves survival rates in women with HR+ breast cancer. Unfortunately, there is significant underuse and lower adherence to AET among black women compared to their white counterparts. AET adherence rates are low for all patients with HR+ breast cancer, most often due to adverse side effects. We will investigate the crucial link between patient symptoms and subsequent treatment course and health outcomes in HR+ breast cancer. Side effects of AET clearly limit medication adherence rates and often trigger clinicians and patients to change or discontinue treatment. To our knowledge, no one has investigated how AET-related side effects and adherence and provider treatment decisions may be driving disparities in B-W health outcomes and mortality. Our study is guided by the Symptom Management Model, which emphasizes the interrelatedness of three symptom management dimensions (symptom experience, management strategies, and health outcomes) while incorporating distinct domains and levels of analyses, including person/health and environment. Our main goal is to understand how B-W differences in potentially modifiable factors in the treatment pathway contribute to B- W mortality disparities. To accomplish this goal, we will pursue the following aims: 1) To determine whether we observe statistically and clinically significant B-W differences (frequency, severity, type) in patient-reported adverse symptoms; 2) To investigate whether black patients and their providers respond differently to the same symptoms in terms of AET adherence and prescribed treatment changes compared to white patients; and 3) To examine if and how patient-reported symptoms, medication adherence, and treatment changes impact B-W disparities in breast cancer outcomes, including mortality, recurrence, and hospitalizations. We will examine patients cared for by the West Cancer Center, an industry leader in leveraging technology to collect and utilize patient-reported outcomes in real-time clinical settings and the largest cancer care provider in the West Tennessee region. This region has the highest B-W breast cancer mortality disparity of the 50 largest US cities. We will leverage nine years of data from multiple sources, including patient-reported measures, clinical data, and claims records for Medicare and Medicaid beneficiaries. Our results will provide actionable mechanisms for targeted interventions to reduce B-W disparities in breast cancer.